U.S. patent application number 13/468905 was filed with the patent office on 2013-05-16 for phase-locked-loop with quadrature tracking filter for synchronizing an electric grid.
This patent application is currently assigned to Rockwell Automation Technologies, Inc.. The applicant listed for this patent is Ahmed Mohamed Sayed Ahmed, Russel J. Kerkman, Brian J. Seibel, Carlos Rodriguez Valdez. Invention is credited to Ahmed Mohamed Sayed Ahmed, Russel J. Kerkman, Brian J. Seibel, Carlos Rodriguez Valdez.
Application Number | 20130120038 13/468905 |
Document ID | / |
Family ID | 48279994 |
Filed Date | 2013-05-16 |
United States Patent
Application |
20130120038 |
Kind Code |
A1 |
Kerkman; Russel J. ; et
al. |
May 16, 2013 |
Phase-Locked-Loop with Quadrature Tracking Filter for Synchronizing
an Electric Grid
Abstract
Methods and systems for synchronizing an electric grid having
unbalanced voltages are provided. A voltage vector may be filtered
in a quadrature tracking filter (QTF) to generate a quadrature
signal. A phase-locked-loop (PLL) operation may be performed on the
quadrature signal to monitor a voltage vector between the grid and
a connected power converter. The QTF and PLL methods are suitable
for either single-phase applications or n-phase (any number of
phases) applications. A frequency estimator estimates the grid
frequency of the electric grid and outputs the estimated frequency
to the QTF algorithms. The frequency estimator may include a
three-phase phase-locked-loop (three-phase PLL) suitable for
estimating the center frequencies of multiple phases of the
electric grid. The frequency estimator may also include means for
reducing the harmonics in the grid system.
Inventors: |
Kerkman; Russel J.;
(Milwaukee, WI) ; Ahmed; Ahmed Mohamed Sayed;
(Mequon, WI) ; Seibel; Brian J.; (Grafton, WI)
; Valdez; Carlos Rodriguez; (Glendale, WI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kerkman; Russel J.
Ahmed; Ahmed Mohamed Sayed
Seibel; Brian J.
Valdez; Carlos Rodriguez |
Milwaukee
Mequon
Grafton
Glendale |
WI
WI
WI
WI |
US
US
US
US |
|
|
Assignee: |
Rockwell Automation Technologies,
Inc.
Mayfield Heights
OH
|
Family ID: |
48279994 |
Appl. No.: |
13/468905 |
Filed: |
May 10, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61559661 |
Nov 14, 2011 |
|
|
|
Current U.S.
Class: |
327/156 |
Current CPC
Class: |
H03L 7/00 20130101 |
Class at
Publication: |
327/156 |
International
Class: |
H03L 7/00 20060101
H03L007/00 |
Claims
1. A method for synchronizing an electric grid, comprising:
receiving a voltage vector of the electric grid; generating a
quadrature signal of the voltage vector using a quadrature tracking
filter (QTF); performing a phase-locked-loop (PLL) operation on the
quadrature signal to determine a phase angle of the voltage vector;
determining a grid frequency of the electric grid; and applying the
determined grid frequency to algorithms of the QTF.
2. The method of claim 1, wherein receiving the voltage vector of
the electric grid comprises measuring the voltage of voltage drawn
from the electric grid by a single-phase power converter connected
to the electric grid.
3. The method of claim 1, wherein receiving the voltage vector of
the electric grid comprises measuring the voltage of voltage drawn
from the electric grid by a multi-phase power converter connected
to the electric grid.
4. The method of claim 1, wherein the grid frequency of the
electric grid comprises the frequency of the PLL.
5. The method of claim 1, wherein determining the grid frequency of
the electric grid comprises low pass filtering the frequency of the
PLL.
6. The method of claim 1, wherein performing a phase-locked-loop
(PLL) operation on the quadrature signal comprises reducing
harmonics in the quadrature signal generated by the PLL.
7. The method of claim 1, wherein performing a phase-locked-loop
(PLL) operation on the quadrature signal comprises reducing
harmonics in the frequency signal generated by the PLL.
8. The method of claim 1, comprising estimating a frequency of the
grid using a frequency estimator, wherein estimating the frequency
of the grid comprises adjusting a low pass filter in the frequency
estimator and applying the low pass filter on a frequency signal of
the PLL.
9. The method of claim 1, comprising using a three-phase PLL to
determine the grid frequency of the electric grid based on the
voltage vector.
10. The method of claim 9, wherein using the three-phase PLL
comprises determining the frequency of a plurality of phases of the
electric grid.
11. The method of claim 9, wherein using the three-phase PLL
comprises reducing harmonics generated by the grid.
12. A grid system, comprising: an electric grid; a converter
configured to receive voltage from the electric grid; a quadrature
tracking filter (QTF) configured to output a quadrature signal
based on a voltage signal proportional to the voltage received by
the converter from the electric grid; a phase-locked loop (PLL)
configured to determine a phase angle of the grid based on the
quadrature signal from the QTF; and a frequency estimator coupled
to the QTF, wherein the frequency estimator is configured to
estimate a grid frequency of the electric grid and configured to
transmit the grid frequency estimate to the QTF.
13. The grid system of claim 12, wherein the frequency estimator
comprises a low pass filter configured to filter the frequency of
the PLL.
14. The grid system of claim 13, wherein the low pass filter is set
between 50 rad/s and 200 rad/s.
15. The grid system of claim 12, wherein the PLL comprises a
harmonic killer configured to reduce the harmonics generated by the
PLL.
16. The grid system of claim 12, wherein the frequency estimator
comprises a three-phase phase-locked-loop (three-phase PLL)
configured to estimate a center frequency for a plurality of phases
of the electric grid.
17. The grid system of claim 16, wherein the three-phase PLL
comprises a harmonic killer configured to reduce harmonics
generated by the electric grid.
18. The grid system of claim 16, wherein the three-phase PLL is
configured to output the estimated center frequencies for the
respective plurality of phases to the QTF.
19. A frequency estimator in an electric grid system, wherein the
frequency estimator is configured to estimate a grid frequency of
an electric grid in the electric grid system and transmit the
estimated grid frequency to a quadrature tracking filter (QTF) of
the electric grid system, wherein the QTF is configured to input a
voltage vector of the electric grid and output a quadrature signal
to a phase-locked-loop (PLL) configured to estimate a phase angle
of the electric grid.
20. The frequency estimator of claim 19, comprising a low pass
filter configured to filter the frequency of the PLL to estimate
the grid frequency.
21. The frequency estimator of claim 20, comprising a harmonic
killer configured to reduce harmonics in the electric grid
system.
22. The frequency estimator of claim 19, comprising a three-phase
phase-locked-loop (three-phase PLL) configured to estimate center
frequencies of a plurality of phases of the electric grid, wherein
the three-phase PLL is configured to output the estimated center
frequencies to the QTF.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Non-Provisional of U.S. Provisional
Patent Application No. 61/559,661, entitled "A Phase-Locked-Loop
with Quadrature Tracking Filter for Synchronizing an Electric
Grid", filed Nov. 14, 2011, which is herein incorporated by
reference in its entirety.
BACKGROUND
[0002] The invention relates generally to electrical networks, and
more specifically, to methods of synchronizing input signals into
the electrical networks.
[0003] Electric devices may be connected or organized in a network
to enable the transmission of power to the devices, or
communication between the devices. Such a network of interconnected
devices may be described as a grid. For example, an electric grid
may be an interconnected network for delivering electricity from
one or more power generators to the connected devices (e.g.,
customers of the utility company). A power grid may transmit AC
power at a synchronized frequency, amplitude, and/or phase angle to
efficiently connect a large number of power generators and devices.
Synchronized operation of a grid, or portions of a grid, may enable
a pooling of power generation, as well as a pooling of loads to
result in lower operating costs.
[0004] The synchronized transmission of AC power may be beneficial
for efficiently transmitting and/or distributing of power. However,
many factors may disturb the synchronization of a grid. For
example, voltage imbalances, angular frequency variations, and
voltage harmonic distortions may significantly disturb grid
synchronization. In particular, voltage imbalances may be common in
a power grid, as single phase loads of a grid may not be evenly
distributed between the phases of the supplied power and may be
continuously connected and disconnected. Such discrepancies in the
amplitudes, frequencies, and/or phase angles between two parallel
voltages may cause abnormal current circulation within the grid
which may result in a large current imbalance. Imbalanced currents
may stress grid devices, such as AC-DC converters, cycloconverters,
active filters, induction motors, and other energy storage systems
which function to convert and/or transfer power through the grid to
the connected electric devices. Imbalanced current may also stress
grid link inductors and capacitors, and imbalanced current in one
end device of a grid may introduce a torque ripple through the
grid.
[0005] Power converters used in single-phase applications such as
fundamental front end (FFE) regenerative braking may be even more
susceptible to damages or inefficiencies resulting from imbalanced
current, as the DC bus voltage ripple may become higher in
single-phase applications than in three-phase applications due to
the relatively higher input current and high DC bus capacitor
ripple in single-phase applications. Furthermore, the drive may not
be able to operate properly if the current circulating through the
single-phase converter is not controlled.
[0006] Conventional methods of synchronizing a grid include using a
phase lock loop (PLL) having a standard synchronous reference
frame. However, such conventional methods may not be sufficient for
alleviating the effects associated with an unbalanced grid,
particularly the effects associated with single-phase converter
applications. Methods of decreasing the effects of voltage and/or
current imbalance in a grid that employs single-phase applications
may improve the performance and synchronous operation of the
grid.
BRIEF DESCRIPTION
[0007] One embodiment relates to a method for synchronizing to an
electric grid. The method includes receiving a voltage vector of
the electric grid and generating a quadrature signal of the voltage
vector using a quadrature tracking filter (QTF). The method also
includes performing a phase-locked-loop (PLL) operation on the
quadrature signal to determine a phase angle of the voltage vector.
Further, the method includes determining a grid frequency of the
electric grid and applying the determined grid frequency to
algorithms of the QTF.
[0008] Another embodiment relates to a grid system. The grid system
includes an electric grid, a converter configured to receive
voltage from the electric grid, a quadrature tracking filter (QTF)
configured to output a quadrature signal based on a voltage signal
proportional to the voltage received by the converter from the
electric grid, and a phase-locked loop (PLL) configured to
determine a phase angle of the grid based on the quadrature signal
from the QTF. The grid system further includes a frequency
estimator coupled to the QTF. The frequency estimator is configured
to estimate a grid frequency of the electric grid and configured to
transmit the grid frequency estimate to the QTF
[0009] In another implementation, a frequency estimator in an
electric grid system. The frequency estimator is configured to
estimate a grid frequency of an electric grid in the electric grid
system and transmit the estimated grid frequency to a quadrature
tracking filter (QTF) of the electric grid system. The QTF is
configured to input a voltage vector of the electric grid and
output a quadrature signal to a phase-locked-loop (PLL) configured
to estimate a phase angle of the electric grid.
DRAWINGS
[0010] These and other features, aspects, and advantages of the
present invention will become better understood when the following
detailed description is read with reference to the accompanying
drawings in which like characters represent like parts throughout
the drawings, wherein:
[0011] FIG. 1 is a block diagram illustrating a power grid system,
in accordance with one embodiment of the present techniques;
[0012] FIG. 2 is a schematic diagram representing a single-phase
power converter, in accordance with one embodiment of the present
techniques;
[0013] FIG. 3 is a schematic diagram of a grid system using a
quadrature tracking filter (QTF) and a phase-locked loop to monitor
the voltage vector of a single-phase converter, in accordance with
one embodiment of the present techniques;
[0014] FIG. 4 is a block diagram representing a phase-locked-loop
(PLL) in the grid system of FIG. 3 which may be used to monitor a
voltage vector of a single-phase converter in the grid system, in
accordance with one embodiment of the present techniques;
[0015] FIG. 5 is a schematic diagram of a grid system using a
frequency of the PLL as an input to the QTF, in accordance with one
embodiment of the present techniques;
[0016] FIG. 6 is a schematic diagram of a grid system using a
frequency estimator to estimate a grid frequency as an input to the
QTF, in accordance with one embodiment of the present
techniques;
[0017] FIG. 7 is a schematic diagram of a grid system using a PLL
with a harmonic killer and a frequency estimator to estimate a grid
frequency as an input to the QTF, in accordance with one embodiment
of the present techniques; and
[0018] FIG. 8 is a schematic diagram of a grid system using a
three-phase PLL to estimate frequencies for three phases of the
electric grid as inputs to the QTF, in accordance with one
embodiment of the present techniques.
DETAILED DESCRIPTION
[0019] An electric grid typically includes a network of loads
(e.g., motors, end devices, etc.) which may be interconnected to
enable communication between the loads and/or transmission of power
to the loads. For example, an industrial grid may include power
generators which generate power to be distributed to various motors
or other devices powered via the grid. A grid may include
alternating current (AC) power sources operating in parallel. Power
generated and distributed by various sources (e.g., a power plant,
a generator, etc.) may be synchronized in frequency, amplitude,
and/or phase angle. Synchronization of AC power may result in the
efficient transmission and/or distribution of power.
[0020] One example of an electric grid system 10 may be illustrated
in FIG. 1, where a generator 12 may deliver power through the grid
system 10 to the motors 18 of different devices. The system 10 may
include a transformer 14, which may control voltages used for
delivering power. As each device may operate on different speeds,
the grid system 10 may also include adjustable speed drives (ASDs)
16 configured to adjust the operating speed of the motors 18 for
each device.
[0021] A typical electric grid system may not always operate
balanced, as the loads (e.g., the devices connected to the motors
18) may not be evenly distributed between phases. The connection or
disconnection of any motor 18 may also affect the signals
distributed by the generator 12 and the three-phase transformer 14.
Furthermore, a voltage imbalance at one motor 18 may affect the
synchronization of other motors 18 coupled to the unbalanced motor
18. For example, at the point of common coupling 20, voltage
imbalance at a motor 18 coupled to ASD1 may also result in voltage
imbalance at a motor 18 coupled to ASD2 or ASD3, as there is no
impedance between the motors 18 to prevent the voltage imbalance
from propagating through commonly coupled motors 18 of the grid
system 10. As discussed, such imbalance (i.e., discrepancies in the
amplitudes, frequencies, and/or phase angles) of the two coupled
voltages may cause a large current imbalance, which may damage both
systems.
[0022] Furthermore, while a three-phase transformer is represented
in the illustrated grid system 10, the present techniques may be
suitable for single phase applications or applications having
different numbers of phases. Power converters used in single-phase
applications may include, for example, fundamental front end (FFE)
regenerative braking, photovoltaic systems, or residential systems.
Due to the nature of single-phase power converter systems, voltage
imbalances in a single-phase converter may be particularly damaging
to the grid system. Moreover, single-phase systems may be more
difficult to monitor using standard PLL techniques.
[0023] An example of a single-phase power converter is illustrated
in FIG. 2. In some embodiments, a grid system 10 may be connected
to a converter 22 having a single phase power source 24. Typically,
a phase-locked loop (PLL) is utilized for controlling phase
synchronization of a grid by regulating to zero the difference
between the PLL output 6' and the phase .delta. of two measured
inputs A sin .delta. and A cos .delta.. However, in single-phase
systems, a single-phase voltage vector may not be monitored with
sufficient accuracy using a two-input PLL.
[0024] In some embodiments, line synchronization system using a
quadrature tracking filter (QTF) with a PLL may be suitable for
tracking a single-phase system and/or for individually tracking a
single phase in a multi-phase system. FIG. 3 is a block diagram of
a line synchronization system 30 having a grid 32 connected to a
single-phase converter 22. In some embodiments, the converter 22
may draw AC power from the grid 32. The line synchronization system
30 may also include a suitable voltage measurement device 36 which
outputs a voltage measurement 38 to the QTF 40. The QTF 40 may
output a quadrature voltage set 42 from the single-phase voltage
measurement 38, and this quadrature voltage set 42 may be input to
the PLL 44, which may output an angle signal 46 based on the
quadrature voltage set 42. The angle signal 46 may be transmitted
to a converter controller 48 which may generate a control signal 50
based on the angle signal 46 to control operations of the converter
22.
[0025] In some embodiments, the QTF 40 may be replaced by a
quadrature signal generator using a suitable transformer, such as a
Hilbert transformer, or a transport delay block suitable for
shifting the phase of the voltage measurement 38 by 90 degrees with
respect to the fundamental frequency of the input signal, thereby
creating the quadrature voltage set 42. The QTF 40 may generate the
quadrature voltage set 42 using any suitable algorithm which may
output a multi-phase quadrature voltage. Furthermore, the QTF 40
may generate the quadrature voltage voltage set 42 from a
single-phase voltage input, or from an n-phase (any number of
phases) voltage input. Equation (1) below represent a single input,
multiple output quadrature tracking filter:
[ x 1 ( t ) x 2 ( t ) ] = [ - a - .omega. 0 .omega. 0 0 ] * [ x 1 (
t ) x 2 ( t ) ] + [ a 0 ] * u ( t ) equation ( 1 ) ##EQU00001##
Equation (2) represents a multiple input, multiple output
quadrature tracking filter:
[ x 1 ( t ) x 2 ( t ) ] = [ - a 0 0 0 ] * [ x 1 ( t ) x 2 ( t ) ] +
[ a - x 1 ( t ) x 2 ( t ) 0 ] * [ u ( t ) .omega. 0 ( t ) ]
equation ( 2 ) ##EQU00002##
For example, the single input, multiple output quadrature tracking
filter represented by equation (1) may be suitable for inputting a
frequency of the voltage vector and outputting a quadrature signal
having frequency and voltage components. The multiple input,
multiple output quadrature tracking filter represented by equation
(2) may be suitable for inputting a frequency and a voltage of the
voltage vector and outputting a quadrature signal having frequency
and voltage components.
[0026] Furthermore, in both the single input, multiple output QTF
of equation (1) and the multiple input, multiple output QTF of
equation (2), two implementations may be used. The two
implementations for each of the tracking filters in equations (1)
and (2) are represented in equations (3) and (4) below:
[ y .alpha. ( t ) y .beta. ( t ) ] = [ 1 0 0 1 ] * [ x 1 ( t ) x 2
( t ) ] equation ( 3 ) [ y .alpha. ( t ) y .beta. ( t ) ] = [ 1 0 0
- 1 ] * [ x 1 ( t ) x 2 ( t ) ] equation ( 4 ) ##EQU00003##
As indicated in equations (3) and (4) above, the sign on the second
state is positive in equation (3) and negative in equation (4), as
the reference frames used in the transformations may have two
different rotations and different possible convergences. In some
embodiments, the QTF 40 may store algorithms to perform either
single input, multiple output quadrature tracking, or multiple
input, multiple output quadrature tracking, depending on the inputs
received. In some embodiments, the QTF 40 may include processing
components for determining the appropriate algorithms to apply on
the voltage measurement 38, or the QTF 40 may include processing
components for applying all algorithms on the voltage measurement
38 and determining suitable outputs as the quadrature voltage set
42.
[0027] As illustrated in FIG. 3, once a quadrature voltage set 42
is generated by a quadrature tracking filter 40, the quadrature
voltage set 42 may be input to a phase-locked loop 44. One example
of a synchronization technique using a PLL 44a and its
corresponding reference frame 70 is represented in FIG. 4. While a
grid may supply voltage in three one phase or in any number (n) of
phases, the phases may be depicted as having a quadrature voltage
input (e.g., V.sub..beta.) from the QTF 40 and an internally
generated signal (e.g., V.sub..alpha.). The two inputs may be in
the form of sinusoidal waveforms which are 90.degree. out of phase,
rotating in steady state, and at the frequency of the grid
voltage.
[0028] The instantaneous angular position .delta. of the equivalent
vector to the phase voltages of the grid may be regulated to a
feedback loop which regulates the voltage in the d-axis (V.sub.d
60), or the sum of the inputs via adder 58, to the value of the
reference signal frequency (e.g., zero in this case).
Alternatively, in some embodiments, the feedback loop may regulate
the voltage in the q-axis to a reference value of one if a per-unit
value is considered. Using the d-axis regulation as an example, the
detected d-component of the voltage vector V.sub.d 60 may also be
referred to as an error signal. V.sub.d 60 may be transmitted to a
compensator 62 which determines a frequency estimate .omega. of the
grid voltage. The frequency estimate .omega. may then be integrated
by an integrator 64 to determine a phase angle estimate .delta. of
the grid voltage. The phase angle .delta. may be used by another
transformation 66 to output a sinusoid and a cosinusoid 68, which
may be fed back and multiplied with the original inputs -A sin
.theta. and A cos .theta. (which is depicted as having an amplitude
of 1 after normalization of the gain 68) at the multipliers 54 and
56 to generate, when added, a new error signal V.sub.d 60.
[0029] For certain operations and applications of the grid system
30, the line frequency of the grid 32 may be tracked. In one
embodiment, an estimate of the frequency .omega..sub.e may be fed
to the quadrature tracking filter directly from the frequency of
the phase-locked-loop. as illustrated in FIG. 5, the grid system 30
involves feeding the estimated frequency .omega..sub.e from the PLL
44 directly back to the QTF 40 and using the .omega..sub.e in the
QTF algorithm (e.g., equations (1) and/or (2)).
[0030] To improve the stability of the grid system 30, in some
embodiments, a frequency estimator may be used to estimate the
frequency of the grid. For example, as illustrated in FIG. 6, the
grid system 30b includes a frequency estimator 76 suitable for
estimating the grid frequency in the system 30. In some
embodiments, the frequency estimator 76 may include one or more low
pass filters which filter the frequency of the PLL 44 and feed the
estimated grid frequency 74 back to the QTF 40. The QTF 40 may
continuously apply updated grid frequencies 74 from the frequency
estimator 76 in the QTF algorithms. In some embodiments, the
frequency estimator 76 may improve the stability of the system 30b
by adjusting the bandwidth of its low pass filter. In one
embodiment, the frequency estimator 76 uses a low pass filter of
approximately 50 rad/s. In another embodiment, the frequency
estimator 76 uses a low pass filter of approximately 200 rad/s.
[0031] In another embodiment, as illustrated in FIG. 7, a grid
system 30c includes a PLL with a harmonic killer (referred to as
PLL/HK 78) with a frequency estimator and low pass filter (referred
to as .omega..sub.e/LPF 80). The harmonic killer in the PLL/HK 78
may regulate and/or minimize harmonic disturbances by reducing the
harmonics generated by the PLL, which may be caused by the discrete
nature of the PLL. In some embodiments, the PLL/HK 78 may output a
frequency signal 83 having reduced harmonics to the
.omega..sub.e/LPF 80. Typically, when harmonic distortion is not
present, a high bandwidth PLL may detect the phase angle and
amplitude of the voltage vector to maintain grid synchronization.
When harmonic distortion is present (e.g., the voltage is distorted
with high-order harmonics), the bandwidth of the PLL may be reduced
to reject and eliminate the effect of the harmonics on the output.
For example, the LPF of the .omega..sub.e/LPF 80 may be reduced to
approximately 50 rad/s to reduce the effect of harmonics on the
output 82. The filtered output .omega..sub.e 84 may then be
transmitted to the QTF 40 which may input the .omega..sub.e into
the quadrature tracking filter algorithms.
[0032] FIG. 8 is a schematic diagram of another embodiment of a
grid system 30c which may be suitable for grids having a
three-phase power supply. The grid system 30c includes a
three-phase PLL with a harmonic killer (referred to as the
three-phase PLL/HK 86) which generates center frequencies in all
phases of the grid 32. Additionally, the harmonic killer in the
three-phase PLL/HK 86 reduces harmonics in the grid 32. In one
embodiment, the three-phase PLL/HK 86 may receive the measured
voltage 38. The three-phase PLL may determine the frequency
.omega..sub.e of the grid while the harmonic killer reduces the
harmonic disturbances generated by the PLL. The harmonic-reduced
frequency .omega..sub.e may be input to the QTF algorithms, which
may generate a quadrature signal based on the input .omega..sub.e
and the voltage measurement 38.
[0033] As discussed with respect to FIGS. 5-8, the present
techniques include many different configurations of the grid system
30 for synchronizing the grid 32 using a quadrature tracking
filter, 40, a phase-locked-loop 44, harmonic killers, frequency
estimators 76, and/or a three-phase phase-locked-loop 86. In
different grid systems 30 and/or for different types of connected
converters 22, different configurations may be appropriate.
Furthermore, the present techniques are suitable for monitoring a
voltage vector of a single-phase power converter or for monitoring
a voltage vector of a multi-phase power converter.
[0034] The configuration of embodiments of the present techniques
of quadrature tracking PLL is not limited to the configurations
illustrated in FIGS. 4-6. Other algorithms or transformations may
be suitable for monitoring a voltage vector having a single phase.
For example, the quadrature set may be algebraically combined to
form balanced three phase sets. Furthermore, the present techniques
may involve additional processing which may be in additional and/or
external devices.
[0035] While only certain features of the invention have been
illustrated and described herein, many modifications and changes
will occur to those skilled in the art. It is, therefore, to be
understood that the appended claims are intended to cover all such
modifications and changes as fall within the true spirit of the
invention.
* * * * *